1. Field of the Invention
The present invention generally relates to semiconductor memories, and more particularly, the present invention relates to method of programming a memory cell array.
A claim of priority is made to Korean Patent Application No. 10-2005-0028086, filed Apr. 4, 2005, in the Korean Intellectual Property Office, the entirety of which is incorporated herein by reference.
2. Description of the Related Art
A phase-change random access memory (PRAM), also known as an Ovonic Unified Memory (OUM), includes a phase-change material such as a chalcogenide alloy which is responsive to heat so as to be stably transformed between crystalline and amorphous states. Such a PRAM is disclosed, for example, in U.S. Pat. Nos. 6,487,113 and 6,480,438.
The phase-change material of the PRAM exhibits a relatively low resistance in its crystalline state, and a relatively high resistance in its amorphous state. In conventional nomenclature, the low-resistance crystalline state is referred to a ‘set’ state and is designated logic “0”, while the high-resistance amorphous state is referred to as a ‘reset’ state and is designated logic “1”.
The terms “crystalline” and “amorphous” are relative terms in the context of phase-change materials. That is, when a phase-change memory cell is said to be in its crystalline state, one skilled in the art will understand that the phase-change material of the cell has a more well-ordered crystalline structure when compared to its amorphous state. A phase-change memory cell in its crystalline state need not be fully crystalline, and a phase-change memory cell in its amorphous state need not be fully amorphous.
Generally, the phase-change material of a PRAM is reset to an amorphous state by heating the material in excess of its melting point temperature for a relatively short period of time. On the other hand, the phase-change material is set to a crystalline state by heating the material below its melting point temperature for a longer period of time. In each case, the material is allowed to quickly cool to its original temperature after the heat treatment.
The speed and stability of the phase-change characteristics of the phase-change material are critical to the performance characteristics of the PRAM. As suggested above, chalcogenide alloys have been found to have suitable phase-change characteristics, and in particular, a compound including germanium (Ge), antimony (Sb) and tellurium (Te) (e.g., Ge2Sb2Te5 or GST) exhibits a stable and high speed transformation between amorphous and crystalline states.
In addition to the differing resistive characteristics mentioned above, the phase-change material exhibits a relatively low reflexibility in its amorphous state, and a relatively high reflexibility in its crystalline state.
FIGS. 1A and 1B illustrate a memory cell 10 in a ‘set’ state and in a ‘reset’ state, respectively, and FIG. 2 is an equivalent circuit diagram of the memory cell 10 of FIGS. 1A and 1B. As shown, the memory cell 10 includes a phase-change element C and an access transistor 20 connected in series between a bit line BL and a reference potential (e.g., ground). Also, as shown, a gate of the access transistor 20 is connected to a word line.
It should be noted that the structure of the phase-change element C is presented as an example only, and that other structures may be possible. Similarly, the connections illustrated in FIGS. 1A, 1B and 2 are presented as examples only, and other configurations are possible. For example, the memory cell 10 may include the phase-change element C and a diode (not shown) connected in series between the bit line BL and the word line WL.
In each of FIGS. 1A and 1B, the phase-change element C includes a top electrode 12 formed on a phase-change material 14. In this example, the top electrode 12 is electrically connected to a bit line BL of a PRAM memory array (not shown). A conductive bottom electrode contact (BEC) 16 is formed between the phase-change material 14 and a conductive bottom electrode 18. The access transistor 20 is electrically connected between the bottom electrode 18 and the reference potential, and the gate of the access transistor 20 is electrically connected to a word line WL of the PRAM cell array (not shown).
In FIG. 1A, the phase-change material 14 is illustrated as being in its crystalline state. As mentioned previously, this means that the memory cell 10 is in a low-resistance ‘set’ state or logic 0 state. In FIG. 1B, a portion of the phase-change material 14 is illustrated as being amorphous. Again, this means that the memory cell 10 is in a high-resistance ‘reset’ state or logic 1 state.
The set and reset states of the memory cell 10 of FIGS. 1A and 1B are establish by controlling the magnitude and duration of current flow through the BEC 16. That is, as shown in FIG. 2, the memory cell 10 is activated (or accessed) by operation of the access transistor 20 which is responsive to a voltage of the word line WL. When activated, the phase-change element C is programmed according to the voltage of the bit line BL. More specifically, the bit line BL voltage is controlled to establish a programming current ICELL which causes the BEC 16 to act as a resistive heater which selectively programs the phase-change material 14 in its ‘set’ and ‘reset’ states. This is explained in more detail below with reference to FIG. 3.
FIG. 3 illustrates an example of temperature pulse characteristics of phase-change material as the phase-change material is programmed in the ‘set’ and ‘reset’ states. In particular, reference number 35 denotes the temperature pulse of the phase-change material programmed to its ‘reset’ state, and reference number 36 denotes the temperature pulse of the phase-change material programmed to its ‘set’ state.
As shown in FIG. 3, when the phase-change material is programmed to its ‘reset’ state, the temperature of the material is increased above its melting temperature Tm (e.g., 610° C.) for a relatively short period of time, and then allowed to rapidly cool. In contrast, when the phase-change material is programmed to its ‘set’ state, the temperature of the material is increased to below its melting point Tm and above its crystallizing temperature Tx (e.g., 450° C.) for a longer period of time, and then allowed to cool. The temperature range between the melting temperature Tm and the crystallizing temperature Tx is referred to as the “set window”.
FIG. 4 is a diagram showing a set current pulse I_SET and a reset current pulse I_RESET that is applied to the phase-change memory cell in order to achieve the temperature pulse characteristics shown in FIG. 3. More particularly, the reset current pulse I_RESET is applied to achieve the temperature pulse 35 of FIG. 3, and the set current pulse I_SET is applied to achieve the temperature pulse 36 of FIG. 3. As one would expect, a larger current pulse of shorter duration is applied to place the phase-change material in its amorphous state than in its crystallize state.
It turns out, however, that the generation of an appropriate ‘set’ current pulse which reliably crystallizes the phase-change material is easier to accomplish in theory then in practice. The phase-change material is highly sensitive to process conditions, and as a result, the temperature range window associated with the crystallization ‘set’ current pulse is extremely narrow. As a result, the crystallization failure rate during programming of phase-change memory cells can be high.